The present invention relates to measurement and control of substrate temperature. More particularly, the present invention relates to an apparatus and method for controlling the temperature uniformity of a substrate during deposition of a coating thereon.
Various industries employ processes to form a thin layer or film on a solid substrate. For example, the production of semiconductor devices utilizes chemical vapor deposition and other deposition techniques to deposit a variety of materials on a substrate. During production of semiconductor devices, heated substrates, such as planar silicon or gallium arsenide wafers or other suitable materials, are exposed to gases which react to deposit the desired materials on the surface of the wafer. Typically, the deposited materials form epitaxial films which replicate the crystal lattice structure of the underlying wafer.
These coated wafers are then subjected to well known further processes to form semiconductor devices such as lasers, transistors, light emitting diodes, and a variety of other devices. For example, in the production of light emitting diodes, the layers deposited on the wafer form the active elements of the diodes. The thickness, composition and quality of the deposited layers determine the characteristics of the resulting semiconductor devices. Accordingly, the deposition process must be capable of depositing films of uniform composition and thickness on the front face of each wafer. The requirements for uniformity have become progressively more stringent with the use of larger diameter wafers and with the use of apparatus which deposits coatings on several wafers simultaneously.
In a typical prior art deposition apparatus illustrated in FIG. 1, a wafer 10 is mounted in a wafer carrier 12 which, in turn, is mounted on a susceptor 14. The susceptor 14 may be mounted on a rotating support spindle 16, which enables rotation of the wafer carrier. The susceptor 14, the wafer carrier 12 and the wafer 10 typically are located in an enclosed process reactor 18. A heating assembly 20 symmetrically arranged below susceptor 14 heats the susceptor, which thus results in the heating of wafer carrier 12 and wafer 10 mounted thereon. Rotation of the carrier 12 is intended to enhance the temperature uniformity across the deposition area, as well as the uniformity of the source material gases or vapors flowing over the deposition area. As is known in the art, reactants are introduced into the reaction chamber and a film is deposited on the surface of the wafer.
Conventional wafer carriers, such as wafer carrier 12 shown in FIG. 2, include multiple cylindrical pockets 22 on their upper surface for holding the wafers in place as the wafer carrier is rotated during the deposition process. These wafer carriers ordinarily also include an annular flange 24 which is used for lifting and transporting the wafer carrier into and out from the reaction chamber. On their bottom surface, the wafer carriers may include an annular wall 26 for locating and holding the wafer carrier concentrically on the susceptor as the assembly is rotated during the deposition process, and for creating a gap 28 between the upper surface of the susceptor and the lower surface of the wafer carrier, which gap eliminates localized heating of the wafer carrier resulting from points of contact between the wafer carrier and the susceptor, and thus enhances the uniform transfer of heat from the susceptor to the wafer carrier.
It is important that the temperature of the wafer is accurately measured and controlled in a manner that is repeatable, precise and independent of process conditions. It is also important to maintain a uniform temperature over the surface of the wafers being processed, and across the surface of each wafer. Deviation from target process temperatures and wafer temperature non-uniformity of only a few degrees centigrade may result in defects in the devices fabricated from the wafers and rejection of the finished product. The temperature of the wafer surface during processing is typically measured using a pyrometer. A pyrometer is a non-contact measurement device that detects radiation emitted from a surface.
The accuracy of temperature measurement by a pyrometer is highly dependent on surface optical properties, particularly emissivity of the body being measured. Emissivity is a parameter that compares radiation from an actual surface versus the radiation from a xe2x80x9cblack bodyxe2x80x9d or ideal radiating body at the same temperature. Pyrometers are calibrated using a black body radiation source. Emissivity values of semiconductor wafers depend on the materials deposited on the semiconductor wafer, substrate doping, surface roughness, and wafer temperature. Emissivity also depends on the thickness of the film grown on the surface of the wafer, and this emissivity value changes during the deposition process. Accordingly, measurement of semiconductor wafer temperature desirably involves the use of emissivity compensated pyrometers, which are designed to overcome the measurement error caused by wafer emissivity variations.
A need exists for a system which provides a more uniform temperature distribution across the surface of the wafers, such that a more uniform coating can be deposited across the entire surface of each wafer.
The present invention provides a method and apparatus for controlled heating of a substrate. One aspect of the invention includes an apparatus for heating a substrate in a chemical vapor deposition reaction chamber. The apparatus includes a carrier for holding at least one substrate in the reaction chamber, and the carrier includes a first zone and a second zone. The carrier typically has a central axis, and the first and second zone are simply regions of the carrier disposed at different radial distances from the central axis. A rotational drive may be provided for rotating the carrier about the central axis.
The apparatus also desirably includes first and second heating elements arranged to heat the carrier and the at least one substrate. The first heating element is preferably arranged to heat the first zone preferentially, i.e., to apply heat principally to the first zone of the carrier. The apparatus preferably further includes at least one substrate pyrometer directed at a substrate surface for measuring a process temperature. The substrate pyrometers desirably are emissivity-compensated pyrometers so that the process temperature measurement represents substrate temperature independent of substrate emissivity. The apparatus desirably further includes at least two carrier pyrometers, each such carrier pyrometer being associated with one zone of the carrier surface. The at least two carrier pyrometers preferably are non-emissivity compensated pyrometers. Each carrier pyrometer is operative to provide a zone signal representing radiation from the associated zone of the carrier.
Most preferably, the apparatus includes a first comparator connected to the first and second carrier pyrometers. The first comparator is arranged to provide a first difference signal representing the difference between the first and second zone signals. Most preferably, one or more controllers are constructed and arranged to control the operation of the second heating element based at least in part on the process temperature. The controllers also may control the operation of the first heating element based at least in part upon the first difference signal.
Most preferably, the one or more controllers provide separate feedback loops so that the second heating element is controlled based upon the process temperature without reference to the difference signal, whereas the first heating element is controlled based upon the difference signal without reference to the process temperature. The emissivity of the carrier surface is unknown and typically changes during the process. Because the carrier surface typically has diffuse reflectivity, an emissivity-compensated pyrometer cannot be used to correct for emissivity of the carrier. Thus, the zone signals from the carrier pyrometers do not normally provide accurate measurements of the carrier temperature. However, because the emissivity of the carrier surface normally is the same in both zones of the carrier, the difference signal provided by the comparator represents the difference in temperature between the two zones. By minimizing the difference signal, the system assures temperature uniformity across both zones of the carrier, which in turn enhances temperature uniformity of the substrates.
More than two zones can be used. For example, the first zone may be disposed radially inward of the second zone, and the carrier may have a third zone disposed radially outward of the second zone. The apparatus may include a third carrier pyrometer operative to provide a third zone signal which represents radiation from the third zone of the carrier. A second comparator may be arranged to provide a second difference signal representing a difference between the second and third zone signals. The one or more controllers are operative to control the third heating element based at least in part on the second difference signal.
Another aspect of the invention includes a method of controlling the temperature of a substrate in a chemical vapor deposition apparatus. The method according to this aspect of the invention desirably includes providing a carrier for supporting the substrate in the chemical vapor deposition apparatus. At least one heating element is used to heat the substrate and the carrier to a process setpoint. According to one aspect of the invention, the temperature of the substrate is measured, preferably using an emissivity compensated pyrometer. Indications of a parameter related to carrier temperature are obtained from at least two zones on the carrier. For example, the indications of such parameter may represent radiation intensity from the two zones, measured by non-emissivity compensated parameters. The indications of such parameter in the at least two zones are then compared with one another to obtain a difference signal which is related to the difference in temperature between the two zones. Heat transfer to at least one of the zones of the carrier is adjusted, as, for example by adjusting power input to one or more heating elements, until the difference signal reaches a preselected level. Typically, this preselected level is substantially equal to zero, so that the two zones will settle at the same temperature.
Additional features and advantages of the invention will be set forth in the description which follows. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.